Intermolecular Forces and Vapour Pressure: Can Bees Avoid a Chemical?

Intermolecular forces and vapour pressure help explain how readily a chemical enters the air. Molecular structure can help predict whether bees avoid a chemical, but volatility alone cannot determine repellency. Molecular shape and chemical properties also influence interactions with smell receptors. A honey-bee study provides a useful real-world way to connect these ideas with Class 11 Chemistry and JEE Chemistry reasoning.

What the honey-bee research found

Researchers used molecular descriptors and behavioural data to screen more than 50 million chemical structures computationally. Of 28 selected candidates tested experimentally, 15 reduced honey consumption in laboratory assays. All seven compounds selected for limited field trials repelled freely foraging bees.

These results support the discovery approach, but they do not establish whole-colony protection when the compounds are mixed with commercial pesticides. Reduced honey consumption is a behavioural measurement, not a direct measure of colony survival. The field-tested group was selected from promising candidates; seven successes do not mean every predicted compound will work.

Read the original eLife study: Machine learning of honey bee olfactory behavior identifies repellent odorants in free-flying bees in the field (DOI: 10.7554/eLife.104831). The chemistry lesson is how molecular properties support a prediction that experiments must then test.

Why an airborne odour needs molecules in the vapour phase

For an airborne smell to reach an insect, some molecules must leave their source and travel through the air. A substance can be present near a bee yet release too little vapour to produce the intended response.

Volatility describes the tendency to vaporise. A volatile liquid evaporates readily under the stated conditions. Vapour pressure is a related, more precisely defined equilibrium property: the pressure exerted by the vapour in equilibrium with its liquid or solid at a specified temperature.

Imagine a closed container holding a liquid. Molecules escape from the surface while vapour molecules return to the liquid. At equilibrium, the two rates are equal. Molecules still move between phases; the equilibrium is dynamic.

An open field is not this closed equilibrium container. Wind, temperature, exposed area, formulation and release rate affect the airborne concentration. Vapour pressure helps explain release potential, but it does not alone predict an outdoor repellent’s performance.

Intermolecular forces and vapour pressure: why molecules escape

To enter the vapour phase, a molecule must overcome attractions to neighbouring molecules. Stronger attractions generally reduce vapour pressure at a fixed temperature when other structural factors are comparable. This relationship between intermolecular forces and vapour pressure is the starting point for comparing volatility.

London dispersion forces

All molecules experience dispersion forces. Their strength depends on polarizability and molecular contact. Larger, more polarizable molecules often have stronger dispersion forces. Shape matters too: branching can reduce effective contact compared with a less-branched isomer.

Dipole–dipole attractions

Polar molecules can attract through their permanent dipoles. However, polarity is not the only variable. Comparing a large nonpolar molecule with a much smaller polar molecule may not give the simple ranking expected from polarity alone.

Hydrogen bonding

Alcohol molecules can hydrogen-bond to one another through their O–H groups. Carboxylic acids can form strongly associated dimers, especially in nonpolar environments. These associations often make them less volatile than comparable ethers or hydrocarbons.

An ether has an oxygen atom and can accept hydrogen bonds from water, but it has no O–H bond to donate hydrogen bonds to another ether molecule. Do not confuse hydrogen bonding with water with hydrogen bonding in the pure liquid.

JEE/NEET concept check: which has the greatest vapour pressure?

Compare a carboxylic acid, an alcohol, an ether and a hydrocarbon of broadly comparable size, polarizability and shape, all at the same temperature and in comparable liquid states.

Expected answer: the hydrocarbon generally has the greatest vapour pressure. It has dispersion forces but lacks the permanent-dipole attractions and conventional self-hydrogen bonding of the other examples. The alcohol and acid experience hydrogen bonding; the ether has dipole–dipole attractions but no self-donated O–H hydrogen bonding.

This is a controlled conceptual comparison, not a universal ranking for every molecule in these four families. Molecular mass, branching, polarizability and physical state can change the result. Always read the conditions in an exam question.

Why similar volatility does not mean the same smell or repellency

Entering the air is only one step. An odorant must also interact with receptors. Size, shape, functional groups and charge distribution can affect that interaction. Two molecules can have similar vapour pressures yet produce different behavioural responses.

A very volatile substance may disperse rapidly; a poorly volatile substance may release too slowly. Whether either concentration is useful depends on the receptor response, application conditions and persistence. The research therefore used multiple molecular descriptors and behavioural testing rather than one simple boiling-point rule.

CBSE/JEE/NEET relevance

CBSE Chemistry: connect this example to chemical bonding, polarity, hydrogen bonding, organic functional groups and physical-property reasoning. Use it as enrichment wherever a detailed vapour-pressure treatment is outside the current prescribed unit coverage.

JEE Chemistry: practise comparisons that control molecular size and shape, distinguish different intermolecular forces, and relate vapour pressure to boiling point.

NEET Chemistry: strengthen reasoning about hydrogen bonding, polarity and the physical properties of alcohols, ethers, acids and hydrocarbons. The research method itself is not something students need to memorise for the exam.

For another example of dynamic equilibrium and its practical limits, read our Haber-process explanation of temperature, equilibrium and reaction rate.

Student practice questions with answers

1. Two similar-sized liquids are compared at the same temperature. One has stronger intermolecular attractions. Which generally has lower vapour pressure?

Answer: The liquid with stronger attractions. Fewer molecules escape readily, giving a lower equilibrium vapour pressure under otherwise comparable conditions.

2. Can a pure ether liquid form O–H···O hydrogen bonds between its own molecules?

Answer: No. It has an oxygen acceptor but no O–H donor. It can accept hydrogen bonds from water or an alcohol.

3. Does evaporation stop when liquid–vapour equilibrium is reached?

Answer: No. Evaporation and condensation continue at equal rates, so the macroscopic amounts remain constant.

4. Why does a higher-vapour-pressure liquid usually have a lower boiling point?

Answer: Boiling occurs when the liquid’s vapour pressure equals external pressure. A liquid with higher vapour pressure at a given temperature generally reaches that condition at a lower temperature, for the same external pressure.

5. If two compounds have equal vapour pressure, must they repel bees equally?

Answer: No. Receptor interactions, airborne concentration, stability and experimental conditions also matter. Physical release and biological response are different stages.

Frequently asked questions

Turn this concept into better problem solving

When comparing physical properties, first fix the temperature and identify the dominant attractions. Then check size, shape and polarizability before choosing an answer. If your answer was wrong, record which variable you overlooked rather than memorising a ranking without conditions.

For a practical review method, read How to Analyse Test Mistakes. For further Chemistry teaching, visit The Lalit Mishra Show.

Lalit Kumar Mishra — Chemistry Educator • Student Success Mentor • Author

Lalit Kumar Mishra is the founder of Topper Formula, an education and learning brand. His teaching connects Chemistry concepts with purposeful practice and student success.